396 DR. J. E. GRAY ON THE GENERA OF TURTLES. [Apr. 1, 



I. The Turtles : with the bones of the vertebra and ribs expanded 

 and forming in the adult state a complete bony disk, the bones 

 of the sternum separate, but united by dentated sutures into a 

 dish, and edged with a series of marginal bones. 



Both the back, margin, and sternum are covered with more or 

 less thick, regular, symmetrical, horny plates, as in most other Che- 

 lonians. 



The head is covered with symmetrical shields ; and the nostrils are 

 in the front, just over the upper edge of the upper beak, which has 

 a more or less deep notch for their reception. 



This group contains two very distinct families. 



Fam. 1. Cheloniad^e, Gray, Hand-list Tort. p. 92. 



The head covered with few, regular, symmetrical shields, and with 

 only one superorbital shield on each side, and two shields on each 

 side of the occiput. The beaks horny, the upper one occupying 

 more than half of the lateral margin, with a sharp dilated margin. 

 The lower jaw fitting into the upper, the lower beak being short, 

 and truncated behind, on a level with the central suture or gonyx, 

 and covered with a large elongate horny plate on each side. 



The head has in the central line two nasals, a frontal, a parietal, 

 and two occipital plates. Two temporal plates. The cheeks are 

 covered with several plates, three or four forming the back edge of 

 the orbit, the front upper part of which is covered by the outer edge 

 of the nasals. 



The tympanic cavity in the skull is surrounded by a large smooth 

 concavity, defined by the surface of the temporal bone. 



The hinder central bone of the dorsal disk, even in the young 

 specimens, reaches to the front edge of the hinder central marginal 

 or caudal bone. 



Tribe 1. Carettina, Gray, /. c. p. 92. 



The head produced, compressed, and narrow in front. The lower 

 jaw smooth, even on the edge, and covered with the beak on the 

 outside, and only slightly fitting into the upper beak, which is 

 smooth on its inner surface. The alveolar surface of the upper 

 beak and of the skull beneath it with a broad diverging ridge on 

 the middle of each side, which is separated in the front by a longi- 

 tudinal groove ; that of the lower beak and jaw deeply concave, with 



5. PI. 31. f. 8. Chelonia tenuis. 



The figures of these two species do not afford me the means of determining what 

 species they belong to ; but they are certainly not what I have described as new. 



6. PI. 31. f. 9-11. Euchelys macroj)its, Philippine Islands, is a young Chelonia, 

 very likely the young of C. mydas or C. marmorata. It is not named on the 

 plate. 



7. PI. 32. Actinemys marmorata, young and adult. Puget's Sound, Oregon, 

 and Sacramento. 



This species is very different from the Emys nigra of Hallowell, quoted as a 

 synonym in the text, and is evidently a redescription of my Emys olivacea. 



